Semiconductor laser diodes, manufactured in material systems including atoms found in groups III-V of the periodic system, such as the material systems InP/InGaAsP, InP/InAlGaAs, GaAs/AlGaAs and GaAs/InGaAs, are key components in most fiber optical communication systems. In system applications it is important to have access to lasers having a high optical output power (a high quantum efficiency) and a linear optical output power (P) as a function of injected current (I). A condition of achieving these objects is that an efficient current confinement exists in the laser structure, i.e., that the leakage currents are as small as possible. Current leakage results in a lower quantum efficiency and a curved, thus non-linear, power-current-(P-I)-characteristic.
One of the most common types of semiconductor lasers is the so called BH-laser where BH stands for Buried Heterostructure. A conventional BH-laser is typically constructed as appears from the schematic cross-sectional view in FIG. 1. The cross-section is taken perpendicularly to the propagation direction of light. In the centre of the laser an active region 1 is provided, here assumed to be made of InGaAsP, which at all of its sides is surrounded by a material having a higher bandgap, which in FIG. 1 is assumed to be InP. At the top and bottom portions layers are provided having the opposite type of doping, in FIG. 1 a p-doped InP-layer above the active region 1 and an n-doped layer 5 under the active region 1, respectively. At the sides of the active region 1, at the right and the left hand side thereof, current blocking areas 7 or current blocking layers are provided, also having a higher bandgap than that of the active region 1. At the very top and at the very bottom of the structure ohmic electrical contacts of metal type are provided, not shown, in order to accomplish current injection for operating the laser.
When a suitable current passes through the active region 1, thus, stimulated recombination is obtained in the active region 1, which is surrounded by materials having higher bandgaps. As has already been described, according to the embodiment of FIG. 1, the material above the active region 1 is p-doped, the material under the active region 1 is n-doped, whereas the material in the active region 1 itself usually is non-doped or low-doped. The structure is thus, as viewed in the direction from the top downwards, a PIN-diode in the cross-section I--I, which is perpendicular to the plane of the paper in FIG. 1 and passes centrally through the active region 1. Two reasons exist for confining or burying the active region 1 in materials having higher bandgaps:
1) In order to achieve a current confinement, since the potential energy for injected charge carriers is lower in the active region 1 than in the surrounding layers having higher bandgaps.
2) In order to obtain an optical confinement, since the active region 1 having a small bandgap has a higher refractive index than the surrounding materials having larger bandgaps, so that the active region 1 forms the core of an optical waveguide, the cladding of which is constituted by the surrounding layers.
When a voltage is applied between the exterior electrical contact, not shown, which is connected to the top p-layer 3, and the other exterior electric contact, not shown, which is connected to the bottom layer 5, so that the top layer obtains a higher potential than the lower layers, current will pass through the structure in a direction downwards. Charge carriers, i.e., holes and electrons, are then injected into the active region 1. They recombine both by spontaneous recombination and by stimulated recombination. For a certain lowest current, called the threshold current I.sub.th, the stimulated gain is so large that it exceeds the losses in the laser cavity which result from coupling of light out through the end facets, dispersion of light against imperfections in the waveguide and absorption of light, absorption of free carriers, and other factors. The component will then start lasing and light is emitted therefrom.
The optical output power P as a function of the electric current I through the laser then looks ideally as is illustrated by the curve A in FIG. 2 describing an optical output power increasing linearly dependent on the increase of the current. However, substantially two physical phenomena result in that instead the characteristic often appears as is illustrated by the curve B in FIG. 2, i.e., comprising a curved characteristic, in which the optical output power increases less than linearly as a function of the increase of current from the threshold current I.sub.th :
1) That the chip is heated, i.e., a high temperature exists in the semiconducting plate, in which the laser is made.
2) Leakage currents dependent on the current (or the voltage).
The heating is proportional to V.multidot.1, where V is the electrical voltage drop over the diode and I is the intensity of the electric current through the diode. This thermal effect can be minimized by arranging a good thermal dissipation. The second physical phenomena is heavily dependent on the way how the current blocking layers 7 are configured which surround the active region 1 at the left and the right in FIG. 1.
In U.S. Pat. No. 5,398,255, "Semiconductor laser having buried structure on p-InP-substrate" for Tomoji Terakado, NEC Corporation, a laser of the type described above is disclosed. In this patent the introduction of one or more thin layers having a lower bandgap in the current blocking structure is discussed. The laser is built on a p-InP-substrate comprising a sequence of layers placed thereon in the current blocking region in a direction upwards, including first a p-InP-layer 11, which belongs to the lower cladding of the waveguide, thereafter a p-InP-layer 17, further an n-InP-layer 18, an InGaAsP-layer 20, an n-InP-layer 21, and at the top an additional n-InP-layer 22 which belongs to the upper cladding. Thus the InGaAsP-layer is located at the last pn-junction in the general p-n-p-n-structure as seen in a direction from the bottom and upwards.
In U.S. Pat. No. 4,752,933, "Semiconductor laser" for Uomi et al., Hitachi Ltd., a semiconductor laser built from GaAlAs on n-or p-GaAs is disclosed. It has lowered refractive index regions 13 formed by disordering a superlattice structure by means of diffusing Zn or implanting ions of Be or Si. A current blocking layer is formed by a p-n inverse junction between a lower cladding layer 3 and the disordered regions.